A ferroelectric random access memory (FeRAM) uses a ferroelectric capacitor to store memory cell data. Each memory cell stores a logic state based on electric polarization of the ferroelectric capacitor. The ferroelectric capacitor has a dielectric including a ferroelectric such as PZT (lead zirconate titanate) between two electrodes. When a voltage is applied to each plate of the ferroelectric capacitor, the ferroelectric is polarized in a field direction. A voltage above the coercive voltage changes the polarization state of the ferroelectric capacitor. The ferroelectric capacitor operates with hysteresis, and current flows to the capacitor in accordance with the polarization state. If the applied voltage is greater than the coercive voltage, the ferroelectric capacitor will change the polarization state in accordance with a polarity of the applied voltage. The polarization state can be maintained after removing the power source, resulting in non-volatility. The ferroelectric capacitor changes between the polarization states within a short time, e.g., about 1 ns. The programming time of the ferroelectric memory device is faster than that of most other non-volatile memory devices such as erasable programmable read only memories (EPROMs), electrically erasable programmable read only memories (EEPROMs), and flash EEPROMs.
The performance of an FeRAM degrades due to aging of the ferroelectric capacitor. One major aging mechanism in such ferroelectric capacitors is imprint. Imprint is the enhancement of one stored polarization state and the weakening of the reversed state. In the worst case, due to aging, an imprinted ferroelectric capacitor cannot be reversed, causing the memory cell to fail. Two types of imprint causing the aging effect are static imprint, which is caused by the storage of certain data over an extended period of time, and dynamic imprint, which is caused by the continuous reading and writing back of the same data. It would be desirable to provide a circuit scheme for reducing the effect of both of these imprint aging mechanisms.